1. Field of the Invention
The present invention relates to a variable focal length lens system and an image pickup apparatus, and specifically to a variable focal length lens system and an image pickup apparatus which can be used for a video camera, a digital still camera, or the like while providing an angle view of 70 degrees or more and a zoom ratio of seven or more.
2. Description of the Related Art
The recording method of a camera, which has been known in the art, includes allowing an imaging element using a photoelectric conversion element, such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS), to form an object image on the surface of the imaging element and allowing each of photoelectric conversion elements to convert the light quantity of the subject image formed on the subject image into an electric output.
Attempts have been made to increase the speed of a central processing unit (CPU) and the integration of a storage medium with recent advancement in fine processing technology. Image data of large capacity, which had not been dealt with, can now be processed at high speed. In addition, attempts have been also made to increase the integration of a light receiving element while decreasing the size thereof to record higher space frequency.
However, the high integration and size reduction as described above had caused a decrease in light reception surface of each photoelectric conversion element, causing an increase in influence of noise with a decrease in electric output. Thus, to reduce the influence of noise, there is an attempt to cause an increase in amount of light reaching to the light receiving element by increasing the aperture ratio of an optical system. In addition, there is also an attempt to place a minute lens element referred to as a microlens array in front of each light receiving element.
The microlens array restricts the exit pupil position of a lens system instead of guiding a light flux traveling between the adjacent elements onto the target element. The more the exist pupil position of the lens system approaches the light receiving element, the more the angle of a principal ray reaching to the light receiving element with the optical axis. Thus, an off-axis light flux directing to a screen-surrounding part forms a large angle with the optical axis. As a result, the light quantity necessary for the light receiving element is not obtained, thereby causing shortage of light quantity.
In recent years, the needs of users have been diversified as digital cameras have become widely used.
In particular, there is a demand of a small-sized camera with a zoom lens of a high variable magnification ratio (variable focal length lens system, hereinafter, also referred to as a vari-focal length lens system) a zoom lens with a variable magnification ratio of more than seven has been provided, while there is a demand of a small-sized camera with a zoom lens of a high variable magnification ratio (vari-focal length lens system).
The type of a zoom lens which has been commonly used as one with a high variable magnification ratio is one of a so-called “positive-negative-positive-positive” four-group type. The zoom lens of the “positive-negative-positive-positive” four-group type includes a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group, having a positive refractive power. In the zoom lens of the “positive-negative-positive-positive” four-group type, when the positional lens state changes from a wide angle end state with the longest focal length to a telephoto end state with the shortest focal length, the distance between the first lens group and the second lens group increases while the distance between the second lens group and the third lens group decreases. Each of the first to third lens groups moves and the movement of the fourth lens group compensates for a variation in image surface position.
As an example of such a zoom lens of the “positive-negative-positive-positive” four-group type, a zoom lens disclosed in Japanese Published Patent Application No. 2008-146016 has been known in the art.
In recent years, the number of wide-angle zoom lenses with zoom ratios of more than 10 has been increased. As such wide-angle zoom lenses, a so-called “negative precedence type” lens in which the first lens group has a negative refractive power has been used in many cases.
For example, in the case of a zoom lens described in Japanese Published Patent Application No. 2007-94174 includes two lens groups, a first lens group having a negative refractive power and a second lens group having a positive refractive power. These lenses are arranged from the object side to the image side in this order.
In addition, a zoom lens described in Japanese Published Patent Application No. 2008-46208 includes a first lens having a negative refractive power, a second lens group having a positive refractive power, and a third refractive group having a negative refractive power, and a fourth lens group having a positive refractive index, which are arranged from the object side to the image side in this order.
Furthermore, in recent years, an aspheric surface lens has come into popular use and a so-called positive precedence type zoom lens having a first lens group with a positive refractive power has also come into use in many cases.
For example, in a positive-precedence zoom lens described in Japanese Published Patent Application No. 2008-102165, aspherical surface lenses are abundantly used for attaining a wider angle and a higher magnification.